what's the difference between wifi useage and mobile data

[AC Question]

this is because of emailing photo issues

 

[ManiacJoe]

Mobile data uses your carrier's network to transmit the data, just like your cellular phone calls.
Wide coverage, slower data transfer; your carrier sees all and bills for the usage.

Wifi uses your/someone's router to go directly to the internet.
Limited distance, fast data transfer. Free for your usage; data paid for by the router's owner.

Most apps don't care which system is used.
Some apps, like SMS messaging, require mobile data.

Your email program will happily use either system. However, most email apps have options to limit attachments when using mobile data so that you are not billed for it.